The invention relates to a method and a device for removing a plate made from a material that can be heated by electromagnetic induction, generally a metal sheet, and retained on a supporting member by a heat-fusible adhesive substance that will at least soften when heated, i.e. loses its mechanical characteristics to a large extent under the action of heat.
The plate made from a material that can be heated by electromagnetic induction and which can be removed by the method and device proposed by the invention may be a substantially flat or curved plate and may have several flat parts adjoining one or more curved parts, formed by folding or bending, and such a plate is preferably made from metal or metal alloy having good properties for heating by electromagnetic induction.
The adhesive substance retaining such a plate on a supporting member must be one which is heat-fusible in that it will at least soften under the effect of heat and, for the purposes of the invention, this adhesive substance might not or need not necessarily be brought to the liquid state by fusion.
By supporting member is meant any base of any nature, for example metal or composite, which is sufficiently rigid to be capable of supporting a plate as defined above which is retained on this base by a layer or film of a heat-fusible adhesive as defined above and thus having a so-called xe2x80x9cplastic rangexe2x80x9d in its temperature range, which is a minimum of approximately 20xc2x0 C. higher than the range of its vitreous transition temperature, i.e. a temperature range at which the bonding forces of the adhesive substance may be reduced by 80%. The adhesive may be a glue which is initially applied to the base in a gel, paste or liquid state or alternatively a heat-fusible film adhesive, for example a thermoplastic material or even a thermosetting network or alternatively may be a thermoplastic or thermosetting resin in the matrix of a base having a composite structure, for example resin used to impregnate reinforcing fibres of one or more surface layers of a stratified composite structure constituting the supporting member for a plate to be removed using the method and device proposed by the invention.
By way of example, the supporting member may be a second plate or sheet of metal or any other sheet, optionally a coating sheet, as would be the case for example with a member such as an aircraft door, consisting of two metal plates or panels bonded one against the other, the method of the invention enabling one metal plate or panel to be removed with a view to repair without damaging the other, by acting on the heat-fusible adhesive substance between the two plates or panels. The base may also be the core, for example a material with a honeycomb structure, of a sandwich panel, for example the floor of an aircraft, wherein one of the two skins on either side of the core is specifically the metal plate to be removed by means of the method and device proposed by the invention.
The base or supporting member may also be a composite structure constituting the inner core of a rotor blade, for example of a helicopter, this internal core, with the spar or spars and the filler body or bodies of the blade being enclosed by a stratified external shell forming in particular the lower surface and upper surface skin of the blade, which meet along the leading edge and the trailing edge of the blade, in which case the plate to be removed by the method and device proposed by the invention may be a protective cap at the leading edge of the blade.
Although the method and device proposed by the invention may be used in numerous applications, this method and this device are described in a context in which they apply to the removal of a protective cap from the leading edge of a rotor blade, in particular of a helicopter, since this application is of particular interest to the applicant.
It is known that the leading edge of helicopter rotor blades is protected, in particular against erosion and impact by foreign bodies, by a cap which is generally made from metal sheet (titanium or stainless steel; or even nickel or aluminium), bended to produce a substantially U-shaped form and which is bonded onto the underlying structure of the blade, which may be metal and/or composite, so that the leading edge and the lower surface and upper surface of the blade adjacent to it are covered, at least over a part of the blade span. During the service life of the blade, it is necessary to remove the protective cap from the leading edge to replace it with a new cap, generally as a result of three types of circumstances: as a result of damage to the blade due to impacts on its leading edge and its protective cap; as a result of damage to the protective cap due to erosion; and, in the case of a composite blade, when superficial layers of the composite structure of the blade have started to come unstuck (peeling of the stratified lower and upper surface skin in the region of the leading edge).
At present, the protective cap is manually stripped from the leading edge in a manner that will be described with reference to FIG. 1, which depicts a composite helicopter rotor blade shown by reference 1, at the end of its aerodynamically profiled part, which is attached by a blade neck 2 of varying section to a blade foot 3, having two bores 4 by means of which the blade 1 is joined by two pins to a rotor hub or a linking member for connection to this hub in a known manner. The leading edge 5 of the blade 1 as well as the adjacent parts of the upper surface 6 and the lower surface (not visible in FIG. 1) of the blade 1 are covered by a metal cap 7 which protects the leading edge, this metal sheet being substantially U-shaped and bonded onto the underlying composite structure of the blade 1 by an adhesive substance as defined above.
The operation of removing the cap 7 firstly consists in using a tool of the wood shears type to make a nick or cut to initiate the removal at the start of the operation, guided substantially along the length of the cap 7, i.e. along the span of the blade 1, starting at a transverse edge (along the chord of the blade 1) of the cap 7, for example from the edge 8 of the cap 7 turned towards the blade foot 3 so as to create a narrow tongue which will be the start of a longitudinal strip 9 of the cap 7, and then inserting the end 10 of this strip 9 into the slot of the shaft 12 of a tool 11 similar to a key such as used for opening a sardine can, the handle 13 of which is manipulated by an operator who turns the tool 11 on itself, about the longitudinal axis of its shaft 12, so as to wind the strip 9 in a spiral about itself around the shaft 12 of the tool 11, by means of which the operator is thus able to make a mechanical tear of the strip 9 by a cold-peeling process.
This manual tearing operation for cold-peeling a longitudinal strip 9 off the cap 7, performed step by step on adjacent strips to remove the entire cap 7, is a delicate operation endangering the operator because each strip 9 wound in a spiral about the tool 11 of the sardine tin key type can behave in the manner of a spiral spring and can release suddenly, whilst the face of the operator needs to be very close as he visually checks the process of tearing back a strip 9, the metal of this strip 9 being held on the underlying composite structure of the blade 1 by a greater or lesser amount of adhesive as it progresses, and as far as possible without tearing the superficial layers of the stratified composite structure of the shell of the blade 1. Regardless of all the precautions taken during this manual operation, bearing in mind the surface treatments applied to the composite blades during manufacture and the adhesion forces built up by the adhesive substances used, consequential damage is often caused to the superficial layers of the composite shell of the blade 1 underneath the cap 7.
EP-A-0 854 208 also discloses a method and a device for removing a cap or a metal shield to protect the leading edge of a helicopter blade against erosion, where the cap or shield is fixed to the underlying metal or composite structure of the blade by a layer of non-metal adhesive by producing an electric field between the metal cap and an electrode in the presence of an electrolyte between the cap and the electrode, which enables the cap to be removed by an electro-chemical machining process.
The main drawback of this method resides in the use of an electrolyte, either in the form of an electrolytic bath in which the blade is dipped until the cap to be removed is totally submersed, which can be difficult to do without letting the electrolyte come into contact with other parts of the blade which might be metal, thus damaging these parts, or by displacing the electrode, continuously or step by step, mounted on a mobile frame, facing the cap of the blade secured on a stationary frame, supplying electrolyte through orifices provided in the electrode for example, which circulates in the space between the electrode and the cap through an electrolyte supply circuit which is displaced with the gantry and requires means to recuperate and recycle the electrolyte.
Another drawback of these known methods and devices is that the electrode must be of a specific shape with a concave U-shaped recess that will surround and substantially conform to the external shape of the cap and the leading edge of the blade. In addition, since blades, and in particular helicopter rotor blades, have an aerodynamic profile which twists about a longitudinal axis of the blade, the electrode which moves with the gantry must also move on the gantry, depending on the span of the blade, across an arcuate path centred on the axis of the twist but without causing any interference between the surrounding electrode and the surrounded cap.
Another disadvantage of the methods and devices disclosed in EP-A-0 854 208 is the fact that if the underlying structure of the blade is metal and the existing layer of adhesive has any gaps or has metal members inserted through it which are in contact with the metal structure of the blade, the electrochemical machining process can cause damage to the blade structure.
The underlying problem of the invention is to remedy the disadvantages of the above-mentioned methods, tools and devices used to remove protective metal caps from the leading edges of rotor blades and propose a method and a device that will meet the various practical demands more efficiently than those used in the present state of the art.
More generally, the underlying problem of the invention is to propose a method and a device that will enable a plate made from a material that is capable of being heated by electromagnetic induction, as is generally the case with the protective cap used for the leading edge of a blade made from sheeting, either of metal or metal alloy, in particular titanium or stainless steel, if said plate, which may be substantially flat or curved, is retained on a supporting member by an adhesive substance which at the very least softens when sufficiently heated and referred to hereafter as a heat-fusible adhesive in the broadest sense of the term, which is generally also the case with the adhesives used to bond protective caps for leading edges of blades to the underlying metal or composite structures of rotor blades of a helicopter.
To this end, the method proposed by the invention as a means of removing a plate which is capable of being heated by electromagnetic induction and retained on a supporting member by a so-called xe2x80x9cheat-fusiblexe2x80x9d bonding substance, which loses a part of its mechanical characteristics and at least softens under the action of heat, is characterised in that it comprises at least the following steps:
placing at least one electromagnetic inductor facing at least one strip of said plate so as to heat said strip of plate by induction and said adhesive underneath said strip by conduction to a temperature in the plastic temperature range of the adhesive so that said adhesive softens, at least in a layer of adhesive in contact with said strip, and
driving at least one tool for mechanically hot- tearing said heated strip so as to tear said plate from said supporting member in a region in which said adhesive has softened.
The basic principle of the invention is to combine two physical-chemical principles which are, firstly, to use the plate made from a material which can be heated by electromagnetic induction as a heating resistor which can rise in temperature very rapidly in order to heat the adhesive by conduction, at least in a layer of adhesive in contact with the plate, so that this heated adhesive is able to reach a temperature at least approximately 20xc2x0 C. above its vitreous transition temperature within a few seconds, i.e. a temperature located in the plastic range of the temperature range of the adhesive (for example a temperature ranging between about 160xc2x0 C. and about 180xc2x0 C. in the case of what are referred to as 120xc2x0 C. category adhesives or, more generally, in a temperature range between about 50xc2x0 C. and about 350xc2x0 C.), this being a temperature at which the bonding forces of the heated adhesive substance can be reduced by some 80% of the value they exhibit when the adhesive substance is cold or at normal ambient temperature. The second principle applied is that of using a tearing force, preferably constant or continuous, as a means of dislodging the plate from its supporting member by hot-peeling an induction-heated strip of this plate, given that the bonding forces are reduced at least in a layer of adhesive, in contact with this heated strip of the plate, layer of adhesive which is in turn heated by conduction to a temperature located in its plastic range.
In order to apply this dual principle, the method proposed by the invention incorporates a step whereby the plate is heated by induction, being capable of transmitting the requisite thermal power, associated with a step of hot-peeling using a tearing tool so as to produce a sufficient peeling angle (between the plane of a strip of plate or plane at a tangent to this strip of plate still bonded to the supporting member, at the point of the instantaneous tearing action, and the plane of a strip portion or plane at a tangent to a strip portion which has just been torn off, at the tearing off plane) to dislodge said strip of plate from the supporting member.
One of the main advantages of the invention is that advantage can be taken of the speed at which strips of the plate are heated by electromagnetic induction heating, so that adjacent strips of the plate can be mechanically torn off, by hot-peeling for example, and the fact that the conduction heating of the adhesive is limited to a relatively thin layer in contact with the heated strip, without heating the supporting member abnormally to a temperature that would be high enough to damage the supporting member, which might be a composite base forming the core of a rotor blade, for example. Applying induction heating by using an electromagnetic inductor is all the more efficient if the plate is made from a non-magnetic metal, as is generally the case with the protective metal caps used on the leading edges of blades, and has a different conductivity and thermal capacity as compared with the bonding substance and the substance(s) of the supporting member, so that the exchanges of heat for heating a strip of plate by induction and for heating the underlying layer of adhesive by conduction are sufficiently rapid to allow removal by mechanically tearing the heated strip of plate, for example by hot-peeling, with or without removing some of the adhesive substance. In effect, the forces induced by hot-tearing in the adhesive must be reduced to a minimum to avoid overheating the supporting member, in particular the underlying composite structure of the blade, and thus risking damage to this supporting member.
In other words, heating a strip of plate by electromagnetic induction allows a layer of adhesive substance in contact with this strip of plate to be heated quickly enough by conduction to soften this layer of adhesive substance, to the point where it is within its plastic temperature range, but without heating the layer of adhesive substance in contact with the supporting member to a temperature so high that it will be critical to this supporting member.
The interest of the method proposed by the invention and its originality reside in the option of being able to bring the adhesive in contact with a strip of plate to be torn off to a temperature of least resistance of this adhesive due to the very high speed of heating by induction whilst maintaining the interface between the adhesive substance and the supporting member at a temperature below a given threshold, depending on the structure of this supporting member.
For the reasons outlined above, the method proposed by the invention advantageously incorporates at least a step which consists in displacing at least one inductor and at least one tearing tool jointly relative to the plate and the supporting member. This being the case, the tearing tool may be a key of the type such as used with a sardine can, driven in rotation about itself (about the longitudinal axis of its shaft) as this key is simultaneously displaced in conjunction with the inductor, a key of this type being capable of producing an angle appropriate to the hot-peeling process.
In particular, the method may incorporate steps consisting in:
displacing, preferably continuously, at least one inductor facing at least one strip of said plate and
displacing, preferably continuously, at least one tearing tool between said heated strip of plate and said supporting member, i.e. the tearing tool, which may be a wedge-shaped pad or a roller, is displaced on a level with the adhesive substance between the plate and its supporting member.
Advantageously, the method proposed by the invention may also incorporate at least one of the following steps:
a step which consists in controlling the displacement speed of the inductor so as to adapt it to the thermal power needed to impart to the adhesive heated by the plate a temperature located within the temperature range of the plastic range of the adhesive and preferably to interrupt or reduce heating of the plate by the inductor when the difference between the instantaneous speed and a nominal displacement speed of the inductor relative to the plate exceeds a predetermined threshold, allowing safety parameters to be applied to the displacement kinematics,
a step which consists in controlling the displacement speed of at least one inductor relative to said strip of plate at least at one measured temperature, selected from at least one temperature of said heated strip, at least one temperature of said adhesive and at least one temperature substantially at the surface of said supporting member, measured at the interface between said supporting member and said adhesive in order to avoid overheating said supporting member,
a step which consists in controlling the displacement speed of at least one inductor and/or the thermal power applied by said inductor at one temperature, at least, prevailing substantially at the surface of said supporting member so as to interrupt or reduce the heat applied by the inductor when said temperature reaches a predetermined threshold, for the same purpose as that mentioned above,
a step which consists in controlling the position of at least one inductor relative to the facing surface of said plate so as to adapt said position to at least one signal from at least one sensor detecting a position of said inductor relative to said plate in order to optimise the positioning, preferably in three dimensions, of the inductor relative to a strip of plate to be heated,
a step which consists in cooling, preferably continuously, said supporting member at least in the region located to the rear of said tearing tool relative to the direction of displacement of the latter in order to protect the supporting member from heating to a critical degree, by cooling its surface very rapidly, thereby preventing any transfer of heat by conduction to the supporting member, and
a step which, if working on a very wide plate, consists in cutting at least one strip in said plate, prior to tearing said strip after it has been heated, transversely to the length of the strips, for example the protective caps of the leading edge of blades with a wide chord, optionally with an integrated de-icing or anti-icing device, of main helicopter rotors which are heavy or of medium tonnages.
Advantageously, the method includes a process of hot mechanical tearing to hot-peel at least one heated strip.
The method proposed by the invention may also include a step which consists in driving at least one tool which tears by cold-peeling in order to tear off at least one non-heated strip from the plate, for example in at least one curved region of said plate having a low radius of curvature, albeit across only a small distance, in order to start or initiate removal of this strip of plate, for example with a tool similar to a key such as used with a sardine can, as described above.
The invention also relates to a device for implementing the method described above, which is characterised in that it comprises:
at least one stationary frame on which said supporting member fitted with said plate can be fixed,
at least one mobile frame, which can be displaced relative to the stationary frame and bearing at least one mobile inductor and at least one mobile tearing tool, which are displaced respectively opposite at least one strip of said plate and in contact with said strip and substantially along the latter or between said strip and said supporting member when said mobile frame is displaced relative to said stationary frame.
Advantageously, this device also has first means, preferably motor-driven, to control the displacements of said mobile frame with said mobile inductor and said mobile tearing tool on said stationary frame in at least a first direction, substantially across the length of at least one strip of plate to be removed.
Simultaneously and advantageously, the device also has second means, preferably motor-driven, to control the positioning of said inductor and/or said tearing tool by displacement on said mobile frame in at least a second direction, transversely to said first direction, and substantially across the width of said strip of plate.
Advantageously, the device also has third means, preferably motor-driven, controlling the positioning of said inductor by displacing the inductor on said mobile frame in a third direction, substantially perpendicular to the first and second directions in order to adjust the space between said inductor and a strip of plate to be removed.
In order to optimise the relative positioning of the inductor and the tearing tool on the mobile frame, the device may also comprise fourth means, preferably motor-driven, to position the inductor and/or the tearing tool on the mobile frame at least substantially in said first direction in order to adjust the gap between the inductor and the tearing tool and follow the changing contour of the supporting member whilst maintaining the distances between them.
In order to protect the supporting member from any inadvertent overheating, the device may also have cooling means, preferably at least one jet of compressed air, which are driven by said mobile frame in order to cool said supporting member behind the tearing tool relative to the direction of displacement thereof.
Said tearing tool may have a roller which rolls on the supporting member, tearing therefrom a strip of plate that has just been heated, although, as mentioned above, the tearing tool may also have a pad in the form of a wedge pulled by the mobile frame so that it slides in the adhesive substance between the supporting member and a strip of plate to be torn off and/or a key of the type such as used with a sardine can driven in rotation about itself on the mobile frame and optionally co-operating with a roller or wedge pad in order to guarantee a suitable angle for hot-peeling.
The device may also have a least one tool for cutting strips, preferably adjacent, in said plate, said cutting tool also being driven by a frame, optionally the one supporting the inductor and the tearing tool, which is mobile relative to the stationary frame.
In order to operate the appropriate steps whereby the displacement speed and/or the inductor power are controlled, it is of further advantage if the device also has at least one temperature sensor and/or at least one sensor to detect the position of the inductor relative to the plate, the temperature and/or position sensor or sensors being advantageously mounted on the mobile frame.
Finally, the device also advantageously has a monitoring and control unit, which remotely controls at least the power and/or frequency of the electric supply of high frequency current to the inductor and, preferably also, at least one of said first, second, third and fourth means for controlling the displacement of the mobile frame relative to the stationary frame and the positioning of the inductor and/or the tearing tool on the mobile frame, and, optionally, the cooling means and cutting tool.